This invention relates to methods of imaging articles having imagable coatings, for example to make lithographic printing plates or electronic parts, such as printed circuits.
A generally used type of lithographic printing plate precursor (by which we mean a coated printing plate prior to exposure and development) has a radiation sensitive coating applied to an aluminum substrate. A positive working precursor has a radiation sensitive coating, which after imagewise exposure to radiation of a suitable wavelength becomes more soluble in the exposed areas than in the non-exposed areas, in a developer. Only the remaining, image, area of the coating is ink-receptive.
The differentiation between image and non-image areas is made in the exposure process where a film is applied to the printing plate precursor with a vacuum to ensure good contact. The printing plate precursor is then exposed to a radiation source; conventionally this has been a UV radiation source. In the case where a positive printing plate precursor is used, the area of the film that corresponds to the image in the printing plate precursor is opaque so that no light will strike the printing plate precursor, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which becomes more soluble and is removed on development.
In the manufacture of electronic parts such as printed circuits, after exposure to radiation and development, the resist pattern is used as a mask for forming the patterns onto the underlying electronic elements - for example by etching an underlying copper foil. Due to the high resolution demands and the requirements of high resistance to etching techniques, positive-working systems are widely used. In particular, in the main there have been used alkali developable positive working resists mainly composed of alkali-soluble novolac resins.
The types of electronic parts whose manufacture may use a resist include printed wiring boards (PWBs), thick- and thin-film circuits, comprising passive elements such as resistors, capacitors and inductors; multichip devices (MDCs); and integrated circuits (ICs). These are all classified as printed circuits.
Imagable compositions may also be applied to plastics films in order to form masks. The required pattern is formed on the mask, which is then used as a screen in a later processing step, in forming a pattern on, for example, a printing plate or electronic part precursor.
Common to virtually all commercial applications of positive working systems employing UV radiation over several decades have been compositions comprising alkali soluble phenolic resins and naphthoquinone diazide (NQD) derivatives. The NQD derivatives have been simple NQD compounds used in admixture with resins, or NQD resin esters in which the photoactive NQD moiety has been chemically attached to the resin itself, for example by esterification of the resin with an NQD sulphonyl chloride.
U.S. Pat. No. 3,802,885 describes a UV sensitive positive working printing plate containing a naphthoquinone-(1,2)-diazide-(2)-5-sulphonic acid derivative, the printing life of which is said to be improved by the inclusion of a polymeric carboxylic acid. Polymeric carboxylic acids listed are cellulose acetate hydrogen phthalate, collophony-containing resin, carboxyl group containing styrene-maleic acid copolymer, oil-free alkyd resin, fatty acid-free phthalate resin and poly(vinyl hydrogen phthalate). Example 1 of U.S. Pat. No. 3,802,885 describes a number of compositions each containing a polymeric carboxylic acid, a novolac resin and 2,3,4-trihydroxy benzophenone tris-[naphthoquinone-(1,2)-diazide-(2)-5-sulphonate]. Each such composition was tested as a printing plate coating and found to have an estimated life (xe2x80x9crun lengthxe2x80x9d) of more than 200,000 copies. A comparison composition without a polymeric carboxylic acid failed after 20 revolutions due to poor adhesion of the image to the plate surface.
The run length of many printing plates can be significantly increased by subjecting them to a heat treatment step (xe2x80x9cbaking stepxe2x80x9d) after their development. However subjecting developed plates to a baking step is not always desirable or practicable.
As demands on the performance of UV sensitive positive working coatings have increased so NQD technology has become limiting. In addition, digital and laser imaging technology is making new demands on coatings.
We have devised new positive working heat sensitive systems, to meet the new demands. Our new systems and methods are the subject of our patent applications WO 97/39894, WO 99/01796, WO 99/01795, WO 99/08879, WO 99/21715, WO 99/21725 and WO 99/11458.
Heat is delivered to the coatings described by conduction, using a heated body such as a stylus, or by charged particle radiation, or, preferably, by means of infra-red radiation, the coatings then containing suitable infra-red absorbers.
Our new systems are very effective and even without a post-development baking step give good run lengths on printing presses but it would be desirable to improve their resistance to organic liquids. Positive working printing plate coatings often have poor resistance to chemicals used in a press room environment. For example the solvents used to clean certain inks from printing plates after initial printing may degrade the remaining coating, and make re-working and further use impossible. Certain inks and fount solutions may contain organic liquids which attack the coatings. These deficiencies are particularly marked with those coatings containing novolac resins.
It is an object of embodiments of the invention to provide heat sensitive coatings with improved resistance to organic liquids, notably those used in printing processes and in printed circuit board manufacture.
We have devised a method which offers improvement of our new systems mentioned above, such that their coatings continue to show good developability, with heated areas dissolving in aqueous developers and with unheated areas remaining insoluble in such developers, but wherein the coatings have improved resistance to certain organic liquids.
We did not find that polymeric carboxylic acids in general were effective in achieving this improvement. However, to our surprise we found that one particular class of polymeric carboxylic acids were effective.
In accordance with a first aspect of the invention there is provided a film-forming composition comprising a carboxylic acid derivative of a cellulosic polymer, and a compound which absorbs incident radiation in the wavelength range 600-1400 nm and converts it to heat, the composition having the property that when provided as a solid coating on a substrate regions which have been heated imagewise selectively dissolve in an aqueous developer leaving behind regions which have not been heated.
Suitably said remaining unheated regions have good resistance to organic liquids. Preferably the remaining unheated regions are more resistant to organic liquids than the remaining unheated regions of a corresponding coating treated in the same way but not containing the carboxylic acid derivative of a cellulosic polymer.
The composition may be a liquid composition, containing a solvent, or a solid composition, for example a coating on a substrate, the solid composition being formed by the evaporation of the solvent from the liquid composition.
In this specification weight percentages of components are expressed with reference to a solid composition.
The presence of the carboxylic acid derivative of a cellulosic polymer appears to confer upon the compositions improved resistance to certain organic liquids, for example petroleum ethers, alkanediols, for example hexanediol, other glycols, glycol ethers, straight-chain alkanols, for example ethanol, branched alkanols, for example isopropanol and 1-methoxypropan-2-ol, cycloalkanols, for example cyclohexanol, and beta-ketoalkanols, for example diacetone alcohols (ie 4hydroxy4-methyl-2-pentanone). When we refer herein to a composition or coating as being resistant to organic liquids we are referring to a composition or coating which is preferably resistant to organic liquids of at least one of these classes (ie petroleum ethers; glycols and glycol ethers; and alkanols) more preferably to organic liquids of at least two of them; and most preferably to organic liquids of all three of them.
The composition may comprise a resin blend having as one resin component a carboxylic acid derivative of a cellulosic polymer.